Interpretive Summary: As grass seed crop field burning in western Oregon was phased-out, alternative non-thermal practices, such as post harvest straw residue removal or incorporation to the soil, and crop rotations were being developed. There is little information available on the practicality and impacts of non-thermal grass seed cropping systems on soil quality. Consequently, in 1992, the multidisciplinary non-thermal cropping systems project was initiated USDA-ARS (Corvallis, OR) at three diverse sites in the Willamette Valley, Oregon. This platform provided an excellent opportunity to evaluate soil quality indexes which are useful to aid agriculturists to identify early indicators of changes in soil management. Soil carbon is an important factor in soil quality. Understanding how it changes with respect to soil crop rotation or crop residue management is important to maintaining high agricultural sustainability. Our research findings showed that soil C was little affected by crop rotation with different crop species or straw residue removal in grass seed productions systems of western Oregon across a range of soil types differing in drainage and other characteristics. Often it was found that higher soil C was associated with continuous grass rotation, especially if the residue was also chopped and remained on the field, compared with clover, wheat, or meadowfoam crop rotations. This was particularly evident for soil organic matter and related to the soil’s microbiological activity. Perennial grass seed crops of western Oregon maintain high soil C levels sequestered in organic matter compared to many conventional agricultural crop systems nationwide.

Technical Abstract:
As grass seed crop field burning in western Oregon was phased-out, alternative non-thermal practices, such as post harvest straw residue removal or incorporation to the soil, and crop rotations were being developed. There is little information available on the practicality and impacts of non-thermal grass seed cropping systems on soil quality. Consequently, in 1992, the multidisciplinary non-thermal cropping systems project was initiated USDA-ARS (Corvallis, OR) at three diverse sites in the Willamette Valley, Oregon. This platform provided an excellent opportunity to evaluate soil quality indexes which are useful to aid agriculturists to identify early indicators of changes in soil management. The objective of this study was to determine the effects of crop rotation and straw residue management on soil carbon. Data from this study showed that soil C was little affected by crop rotation with different crop species or straw residue removal in grass seed productions systems of western Oregon across a range of soil types differing in drainage and other characteristics. Often it was found that higher soil C was associated with continuous grass rotation, especially if the residue was also chopped and remained on the field, compared with clover, wheat, or meadowfoam crop rotations. This was particularly evident for soil organic matter (SOM). The SOM often increased in the upper soil layer with straw remaining. In all cases, the highest concentration of soil C of the top 20 cm of soil was found in the upper 0 to 10 cm. It was interesting that there was often a decline in soil C from 1992 to 1998. Higher soil c appears to be linked to higher MBC and related to straw remaining in most cases. The Marion site with well-drained soil had the highest MBC compared to the lesser-drained sites of Benton and Linn. Higher DOC was particularly associated with residue additions and may be related to residue C decomposition processes. Perennial grass seed crops of western Oregon maintain high soil C levels sequestered in organic matter compared to many conventional agricultural crop systems nationwide.